In fluid catalytic cracking (FCC), a stream of hydrocarbon feed when contacted with fluidized catalyst in reactor at temperature around 490 to 550° C. is converted into valuable petroleum products. In the process a significant amount of coke gets deposited on the catalyst which makes it unfit for further reaction. However, a major amount of the activity of the spent catalyst can be regenerated for further use by burning off most of the deposited coke in presence of oxygen rich air in regenerator. In this process, coke is converted into carbon monoxide and carbon dioxide. Generally regeneration is carried out at high temperatures of about 600 to 690° C. Higher the temperature more is the removal of coke. After this process the regenerated catalyst is returned to the cracking zone for catalyzing the cracking of heavy hydrocarbons further.
A major disadvantage associated while regenerating FCC catalyst is that of afterburning, which implies further burning of CO to CO2 and takes place in dilute phase. It is advisable to avoid afterburning as it could lead to very high temperatures, which may damage the equipment as well as destroy the catalyst activity permanently.
Furthermore, in recent years, the environment regulations are becoming very stringent with respect to the emission of CO into the atmosphere, which necessitate changes in the functioning of FCC units, irrespective of the costs of petroleum refining. Hence need has arisen to develop a catalyst or additive or a process which can reduce or eliminate afterburning as well as emission of CO into atmosphere.
In processes disclosed in U.S. Pat. Nos. 4,097,535 and 4,093,535, noble metals such as Pt, Pd, have been impregnated in the Y-type zeolite which is the active component of the FCC catalyst. Such catalysts with 25 ppm Pt and 25 ppm Pd though significantly control CO2/CO ratio, suffer from flexibility limitations such as, (1) addition of CO-combustion promoter cannot be terminated as and when required, (2) in order to increase the CO2/CO ratio entire inventory has to be discharged and a fresh, more active composite FCC catalyst has to be loaded, (3) precious metal may be buried by coke in super cages of zeolite and limits platinum's efficacy.
The use of precious metals to catalyze oxidation of carbon monoxide in the regenerators of FCC units has gained broad commercial acceptance. In the earlier stages of development as described in U.S. Pat. No. 4,171,286, the precious metal was deposited on the particles of cracking catalyst. But, the present practice is generally to supply a promoter in the form of solid fluidizable particles containing a precious metal, such particles being physically separate from the particles of cracking catalyst. The precious metal or its compound is supported on particles of suitable carrier material and the promoter particles are usually introduced into the regenerator separately from the particles of cracking catalyst. The particles of promoter are not removed from the system as fines and are circulated along with cracking catalyst particles during the cracking/stripping/regeneration cycles.
U.S. Pat. No. 4,199,435 disclosed a combustion promoter selected from the group of Pt, Pd, Ir, Os, Ru, Rh, Re and copper on an inorganic support. Promoter products as disclosed in U.S. Pat. No. 4,222,856 and used on a commercial basis in FCC units include calcined spray dried porous microspheres of kaolin clay impregnated with a small amount (e.g., 100 to 1500 ppm) of platinum. Most commonly used promoters are obtained by impregnating a source of platinum on microspheres of high purity porous alumina, typically gamma alumina. U.S. Pat. No. 4,544,645 discloses a bimetallic of Pd with every other Group VIII metal but Ru.
U.S. Pat. No. 4,585,752 has mentioned the addition of rare earth metal on Pt based CO promoters. U.S. Pat. No. 4,608,357 refers to palladium based effective CO oxidation promoter. Here, palladium is supported on particles of a specific form of silica-alumina, namely leached mullite.
U.S. Pat. Nos. 5,164,072 and 5,110,780, relate to an FCC CO promoter having Pt on La-stabilized alumina, preferably about 4-8 weight percent La2O3.
U.S. Pat. No. 5,565,399 discloses a novel CO promoter composed of catalytic platinum particles dispersed over alumina support particles and stabilized physically and chemically with a mixture of rare earth for higher catalytic activity, longer catalyst durability and better unit retention of the catalyst in a cracking unit. U.S. Pat. No. 6,117,813 discloses promoter consisting of a Group VIII transition metal oxide, Group IIIB transition metal oxide and Group IIA metal oxide.
U.S. Pat. Nos. 6,165,933 and 6,358,881 describe compositions comprising (i) an acidic oxide support, ii) an alkali metal and/or alkaline earth metal or mixtures thereof, (iii) a transition metal oxide having oxygen storage capability, and (iv) palladium to promote CO combustion in FCC processes while minimizing the formation of NOx.
From the prior art patents U.S. Pat. Nos. 4,542,118, 4,476,239, 4,443,553, EP 0742044 A1 and CA 9,671,36 it can be seen that the flocculating property of alumina is being arrested by the application of aluminium chlorhydrol, aluminum hydroxynitrate in a significant quantity. It may be noted that since aluminum chlorhydrol contains 17% of chlorine therefore its application can corrode the hardware being used for shaping of microspheres. Beside, chlorine and nitrate gases emanating while processing such dispersant loaded products is harmful to atmosphere and is well known for their adverse effects on human health.
It has been disclosed in the prior art (U.S. Pat. Nos. 4,476,239, 4,443,553 and CA Patent 9,671,36) that addition of a small amount of aluminium hydroxy chloride or hydroxy nitrate reduces the viscosity of the slurry to an extent and increases the free flow nature of the slurry for easy spray drying. U.S. Pat. No. 4,542,118 also discloses the preparation of an organic composition from slurry of alumina and aluminium chlorhydrol in order to reduce the viscosity.
EP 18799821 discloses a composition for promoting CO oxidation with controlled emission of NOx. The composition contains alumina as support, ceria, praseodymium oxide, along with transition metals like Cu, Ag, Zn. US 2007/0129234 describes a CO oxidation promoter for FCC which minimizes co-incidental HCN and NH3 oxidation. The catalyst comprising mixture of base metal oxides, possibly containing promoters of Ni, La, alkaline earth, rare earth, etc., on alumina support. The disclosed catalyst has a CO oxidation activity per weight of catalyst less than 10% and at least 0.2% activity for CO oxidation per unit weight of a catalyst comprising 500 ppm platinum on alumina at a temperature of at least 1100° F.
US2009/0050529 teaches a composition for CO oxidation promoter with the basic support material being a hydrotalcite compound. The composition comprises at least one oxide of a lanthanide series element, one oxide of a transition metal preferably from Group IB and IIB of the periodic table and one precious metal, Pt along with hydrotalcite support.
From the above discussion it is clear that with growing regulations on CO emission in the environment and for circumventing the problem of afterburning associated with the FCC technology, improved methods are continuously sought which besides being environmental friendly also provides efficient oxidation of CO.